1. Field of the Invention
The present invention relates to apparatus, software, and methods for filling a mold with a liquid material. More particularly, the present invention relates to apparatus, software, and methods for filling a mold with a liquid material by pulsing the liquid material flow.
2. Description of the Related Art
Injection molding is a method for manufacturing parts using a material that is a liquid in an uncured state, and that may be transformed into a solid by a curing or freezing process within a mold. Two broad classes of molding materials used in injection molding are thermal plastic materials and thermal set materials. Thermal set materials remain liquid at relatively low temperatures, and through application of heat over time (i.e., curing), the chemical composition of thermal set materials can be altered to yield a solid. Thermal plastic materials, on the other hand, are solid at relatively low temperatures, and must be melted before injection into a cool mold. Once cooled, thermal plastic materials re-solidify (i.e., freeze) with little or no alteration to the chemical composition of the material.
Sometimes it is advantageous to simultaneously fill multiple mold cavities with liquid material from a common manifold in a multi-port direct injection molding system. Such a system may benefit from uniformity of flow through the multiple ports to promote the quality of the parts so molded. As discussed above, the curing of thermal set materials and solidification of thermal plastic materials both depend upon heat transfer processes within the mold, and ideally, the molded material begins to cure or solidify once the mold is completely filled. Thus, the heat transfer between the molded material and the mold is ideally tailored to a particular mold filling rate.
However, a mold cavity filling at a rate slower than the corresponding designed heat transfer rate poses a risk that the mold material may begin to solidify before the mold is completely full. This condition of premature curing or solidification is referred to as a “short shot,” and may result in defects in the molded product. In the alternative, a mold cavity filling at a faster rate than the corresponding designed heat transfer rate poses a risk that the mold material may begin to over-pressurize the mold cavity and overflow the cavity. This condition of overflowing the mold cavity is referred to as “flash,” and may result in defects in the molded product. Thus, in a multi-port direct injection molding system, it is desirable to have identical mold cavities fill at identical rates, commensurate with the corresponding designed heat transfer rates, to promote molded part quality.
Some molding material properties pose challenges to flow uniformity in multi-port direct injection molding systems. These problematic material properties include compressibility and non-Newtonian behavior. The density of a compressible liquid is affected by changes in pressure. Thus, a mass of compressible liquid that fills a volume at one pressure may spill over from the volume at a lower pressure or fill less than the full volume at a higher pressure. Further, pressure does not transmit instantaneously through a compressible liquid. Rather, pressure inputs travel through a compressible liquid at a finite wave speed.
A liquid is non-Newtonian when its shear rate is not directly proportional to the shear stress applied. One non-Newtonian fluid characteristic exhibited by some molding materials is shear thinning behavior, where the apparent viscosity of the fluid decreases with increasing shear stress applied. Examples of molding materials that exhibit both compressibility and shear thinning behavior are liquid silicone rubber (LSR), which is a thermal set material, and thermal plastic elastomer (TPE), which is a thermal plastic material.
Shear thinning behavior can be problematic in multi-port, direct injection molding systems because differences in shear energy from flow of the silicone through different channels may cause differences in apparent viscosity among the different channels. In turn, the differences in apparent viscosity among different channels can cause undesirable non-uniformity in flow through the different channels. Further, these differences in apparent viscosity can amplify over time because the channels with the highest flow experience the most shear thinning, which may cause the fluid to flow even faster and cause even more non-uniformity among the mold cavity filling rates.
Control of flow balance among parallel channels in a multi-port system has been attempted by altering the set points of parallel runner heaters between the manifold and the mold cavities. This temperature control approach takes advantage of the physical relationship between the temperature of the molding material and its viscosity to counteract shear thinning behavior.
However, the temperature control method is disadvantageous because one must know the extent of the system flow non-uniformity a priori in order to choose the control heater set points. Further, the thermal capacitance of a multi-runner injection system can result in slow temperature response of the liquid material, thereby causing production delays. Moreover, the temperature control method is not considered applicable to thermal set materials because adding heat to the thermal set material upstream of the mold could cause undesirable material curing upstream of the mold or premature curing within a mold cavity.
Attempts have been made to mitigate flow imbalance in multi-port systems by tailoring the flow resistance of parallel channels. Many of these strategies employ some form of a mechanical choke to bias flow path restrictions in the manifold channels or nozzles. Simple systems employ manual chokes positioned within the manifold flow channels so as to tailor the flow restriction between the pressure source and each runner extending to one or more mold cavities. More advanced systems vary the opening stroke of different valves in order to counteract relative flow variations among different nozzles.
Valve gate control is another strategy for mitigating flow imbalance in multi-port systems. Valve gate control tailors the opening and/or closing times of valves in individual parallel channels according to fixed time schedules or based on a measured input such as a pump impeller position. For mold cavities that fill fastest, delays are added to the opening of the nozzles feeding those cavities.
However, the flow distribution control approaches involving mechanical chokes, differential valve throttling, and differential valve opening delays all suffer from a common disadvantage as differential temperature control, namely that the nature and degree of imbalance in the system must be characterized a priori in order to implement the control. Indeed, trial and error is required to implement correctly sized manual chokes in the correct locations, or to select the correct biases in valve opening positions, or to select the correct valve opening delays to improve the balance a multi-port direct injection molding system. Further, the flow resistance tailoring scheme that improves the balance for one machine paired with one mold may not balance the flows in another machine paired with the same mold or the same machine paired with different mold.
Moreover, attempted solutions that function by throttling the size of flow channels in the manifold or runners of a multi-port injection system create viscosity variation in each flow channel when the fluid has shear thinning characteristics. While flow balance may be improved, the liquid material flowing through the restricted flow paths may be heated as it is restricted, thereby introducing runner-to-runner thermal variations which can lead to undesirable pack characteristics in the molded parts. In addition, the valve gate control systems that delay valve openings for the faster flowing channels may introduce differences in fill times, which can affect shrinkage conditions and therefore part quality.